In this chapter, the people complain and Moses sins in Meribah, Edom resists the passage of Israel, and Aaron dies.
This chapter continues with the story, abruptly ending the Levitical interlude of chapters 18-19. When discussing chapter 17, I said that it ended the "Rebellion Arc", which is true in the sense that they have largely stopped challenging Moses and Aaron, but as we can see they are still complaining and sinning against the LORD. In this chapter, the issue is lack of water, which reminds us that they are, in fact, traveling through a desert.
On a minor note, I find it amusing how the people say "if only we had perished when our brothers perished before the LORD", referring to the deaths of Korah and his followers. This is pretty ironic considering the LORD killed them for their rebellion, so now they are basically saying they wish they had rebelled. This shows us that the survivors still hold Korah, Dathan and Abiram in high regard and honor them in spite of their shameful behavior and subsequent punishment. This is not a good omen for Israel's future.
More significantly, this chapter contains the first and only sin of Moses and Aaron, which is hard to notice at first: Moses strikes the rock with his staff, rather than speaking to the rock. Compare the command in v. 8 with Moses's action in v. 11. Interestingly, the last time Moses was commanded to bring water from a rock, in Ex 17:6, he was told to strike the rock with his staff; in this case, he is told to take his staff and speak to the rock, but in both events he strikes the rock with his staff. Aaron appears to be indicted by complicity with Moses's actions, even though we aren't told of Aaron actually doing anything other than being there and helping to "gather the assembly".
Moses's frustration is palpable, as he verbally reproaches the Israelites. I think this is the first time he has spoken harshly against the people, in spite of how many times they have rebelled against him and the LORD. In many instances, Moses replying to the people and challenging them, but never insulting them. In many other occasions, he asks the LORD for mercy on their behalf, such as Ex 32, Num 12 (praying for Miriam), Num 14, and probably more instances if I searched.
So Moses is getting frustrated, and I can understand why because I'm getting frustrated with the people too. Unfortunately, that frustration leads him to act differently than the LORD commanded him, but it seems like a really minor issue so I wouldn't expect it to be treated severely. And yet it is; because of this misstep, the LORD will no longer bring Aaron and Moses into the promised land: they will die in the wilderness. This is really astonishing for such a small mistake, but I think it is significant because it extends the principle we have seen before in e.g. Lev 21, that those people who dwell in the presence of the LORD are held to higher standards than the people who do not. Priests are restrained from going near dead bodies, but the high priest cannot even rend his garments (as a sign of grief).
The nation is forbidden from entering the promised land because they rebelled against Moses, were going to kill him and return to Egypt. That is a very serious crime. Moses rebelled against the LORD by striking the rock instead of speaking to it, and he is given the same punishment not because his crime is so severe, but because his responsibility and authority is so great.
What makes this story confusing is that the LORD honors Moses by bringing forth water, while at the same time rebuking Moses for disobeying his command. This is surprising, because one would think that the LORD would refuse to perform miracles on Moses's behalf if Moses were disobeying the LORD. And yet, here he does. I'm not sure what to think about this. I know the people needed water; though they are complaining and acting without faith, they are correct in stating their need for water. So I believe the LORD wished to give them water, and he chose Moses to be the conduit for this miracle.
The question is, should Moses's mistake disrupt this plan? More generally, should the sins of people who have been given true assignments by the LORD disrupt or destroy those assignments? Personally, I don't think there is a simple yes or no answer to that. In this case, the answer appears to be no, as the LORD fulfills the miracle despite Moses's sin. On the other hand, Moses and Aaron are henceforth denied entrance to the promised land, which is a far more significant blow to their destinies. So I don't think there's a clear answer. Moses definitely suffers for this sin, but his punishment is not total: he still leads the nation to the border of the promised land and is honored in other ways.
Next, we are told that Moses wishes to lead the people through Edom, on their way towards the promised land. Edom is the nation south of ancient Israel, while to the north and west are the Canaanite tribes. We already knew that the LORD directed the people to take the Red Sea coast, heading south into the Arabian Peninsula. Now we can see they are heading north, out of the Peninsula, and towards the promised land. Strikingly, there has been no mention of 40 years in the wilderness, nor will there be any mention of it until the next census is taken.
This is a really surprising (to me) omission. The 40 years in the wilderness represents the metaphorical and literal death of a generation because they rebelled against the LORD and it is left almost entirely out of this account, which is the primary account of the wilderness travels*. Basically the text goes from the LORD condemning Israel to 40 years of wandering, to a brief Levitical section, to an account of Israel's return to the promised land (beginning in this chapter). My best guess is that the "40 year gap" is between the end of chapter 14/beginning of chapter 15, though it could possibly also be at the end of chapter 17. Both of these are junctures where the text shifts from a narrative section to a legal section, and whenever you see a sudden shift in the subject or context of the passage, that is a hint that some sort of temporal discontinuity just occurred. Chapter 15 begins with laws dealing with the promised land itself (see Num 15:2, "when you enter the land where you are to live..."), which I remarked at the time was very peculiar to be positioned directly after the judgment of chapter 14.
One plausible explanation is that these are not only different subjects, but they are recorded at very different times. Perhaps chapter 15 happens after the 40 years, and is therefore more suitable for the grand entrance to Canaan, rather than the humiliating departure. This would require the account of Korah's rebellion et al. to also be some arbitrary time into, or just after, the 40 years of wandering. In my opinion, that is also plausible, since we aren't given any definitive connection between Korah's rebellion and the stories arcs before or after it. That said, Korah's rebellion is topically related to what happens before the 40 years of wandering, in particular the rebellion of chapter 14 itself, since all of these events show the stubbornness of the Israelite nation.
It's also possible the "40 year gap" is at the end of chapter 17/beginning of chapter 18, when the next major Levitical section occurs. I think this is plausible, but I think the 14/15 disjunction has more explanatory power, because reading chapters 14 and 15 as a continuous narrative is simply disconcerting.
Anyway, Edom denies Israel passage because they fear that Israel will 1) consume all the food and water in the land, 2) Maybe leave a few hundred thousand people behind to colonize the land and drive out its current occupants. Remember that Edom is the nation descended from Esau, who was also called Edom. That's why v. 14 says "your brother Israel", because it's speaking of the brotherhood of Jacob and Esau. Now the nations Israel and Edom are also metaphorically brothers.
In the end, Edom does not trust Israel to hold to their promises, and while Edom is rebuked for this in later parts of the bible, I think their fear is justified. Just look at how many times the Israelites have complained to Moses about lacking food or water. It is unrealistic to expect the people to avoid eating any crops or drinking water as they pass through, and while Moses offers to pay for the water, the Edomites similarly distrusted that promise. The brotherhood of Israel and Edom is why the Israelites refused to fight and moved on when "Edom came out against [Israel] with a heavy force and with a strong hand." It's not out of fear, it is to avoid conflict with what should have been a friendly nation. Maybe the Edomites remember the treachery of Jacob and impute that treachery to his children.
At the very end of the chapter, Aaron dies, as we knew he would. Eleazar is to be anointed the next high priest, which makes sense because he is the oldest living son of Aaron. Also, chapter 19 implied that Eleazar would be the next high priest when the LORD commanded that he officiate over the red heifer sacrifice. All things considered, this is a very orderly transition and it effectively seals the hereditary office in Aaron's bloodline.
*There is an ex post facto description of the 40 years in Deuteronomy, when Moses recounts the history of Israel, but Numbers purports to be a present-tense narrative account.